Imaging with gate controlled charge storage

ABSTRACT

A pixel cell comprises a photo-conversion device for generating charge and a gate controlled charge storage region for storing photo-generated charge under control of a control gate. The charge storage region can be a single CCD stage having a buried channel to obtain efficient charge transfer and low charge loss. The charge storage region is adjacent to a gate of a transistor. The transistor gate is adjacent to the photo-conversion device and, in conjunction with the control gate, transfers photo-generated charge from the photo-conversion device to the charge storage region.

FIELD OF THE INVENTION

The present invention relates to the field of semiconductor devices,particularly to an improved pixel cell for efficient charge transfer andlow charge loss.

BACKGROUND OF THE INVENTION

Complementary metal oxide semiconductor (CMOS) image sensors areincreasingly being used over charge coupled device (CCD) image sensorsas low cost imaging devices. A typical single chip CMOS image sensor 199is illustrated by the block diagram of FIG. 1. Pixel array 190 comprisesa plurality of pixels 200, which are described below, arranged in apredetermined number of columns and rows.

Typically, the rows of pixels in array 190 are read out one by one.Accordingly, pixels in a row of array 190 are all selected for readoutat the same time by a row select line, and each pixel in a selected rowprovides a signal representative of received light to a readout line forits column. In array 190, each column also has a select line, and thepixels of each column are selectively read out in response to the columnselect lines.

The row lines in pixel array 190 are selectively activated by a rowdriver 191 in response to row address decoder 192. The column selectlines are selectively activated by a column driver 193 in response tocolumn address decoder 197. The pixel array is operated by the timingand control circuit 195, which controls address decoders 192, 197 forselecting the appropriate row and column lines for pixel signal readout.

The signals on the column readout lines typically include a pixel resetsignal (V_(rst)) and a pixel image signal (V_(sig)) for each pixel. Bothsignals are read into a sample and hold circuit (S/H) 196 in response tothe column driver 193. A differential signal (V_(rst)-V_(sig)) isproduced by differential amplifier (AMP) 194 for each pixel, and eachpixel's differential signal is amplified and digitized by analog todigital converter (ADC) 198. The analog to digital converter 198supplies the digitized pixel signals to an image processor 189 which canperform appropriate image processing before providing digital signalsdefining an image.

An electronic shutter for image sensors has been developed to serve inplace of a mechanical shutter. The electronic shutter controls theamount of photo-generated charge accumulated by a pixel cell bycontrolling the integration time of the pixel cell. This feature isespecially useful when imaging moving subjects, or when the image sensoritself is moving and shortened integration time is necessary for qualityimages.

Typically a pixel cell having an electronic shutter includes a shuttertransistor and a storage device, which is typically a pn-junctioncapacitor. The storage device stores a voltage representative of thecharge generated by a photo-conversion device in the pixel cell. Theshutter transistor controls when and for how long charge is transferredto the storage device and therefore, controls the integration time ofthe pixel cell.

There are two typical modes of operation for an electronic shutter:rolling and global. When an electronic shutter operations as a rollingshutter, each row of pixels in an array integrates photo-generatedcharge one at a time, and each row is read out one at a time. When anelectronic shutter operates as a global shutter, all pixels of an arrayintegrate photo-generated charge simultaneously, and each row is readout one at a time.

Global shuttering provides advantages over row shuttering. Essentially,global operation is able to provide a “snap shot” of the imaged subject.Consequently, global operation offers increased accuracy of an imagedsubject and a uniform exposure time and image content.

On the other hand, because the pixel cells of the pixel array are readout row by row, pixel cells in a row which is read out last must storephoto-generated charge in their respective storage devices longer thanpixel cells in earlier read rows. The conventionally used storagedevices may lose charge over time, and the longer the conventionalstorage devices must store photo-generated charge, the more charge islost. Therefore, charge loss is especially problematic for pixel cellsin a last read row. When charge is lost by a pixel cell, the resultantimage may have a poor quality or be distorted.

Additionally, in conventional pixel cells, potential barriers may existin the path of the photo-generated charge as it is transferred from thephoto-conversion device to readout circuitry. Such potential barriersmay prevent a portion of the photo-generated charge from reaching thereadout circuitry, thereby reducing the charge transfer efficiency ofthe pixel cell and also reducing the quality of a resultant image.Accordingly, what is needed is a pixel cell with an electrical shutterhaving improved charge transfer efficiency and minimal charge loss.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the invention provide an improved pixel cell withincreased charge transfer efficiency and low charge loss. A pixel cellcomprises a photo-conversion device for generating charge and a gatecontrolled charge storage region for storing photo-generated chargeunder control of a control gate. The charge storage region is adjacentto a gate of a transistor. The transistor gate is adjacent to thephoto-conversion device and, in conjunction with the control gate,transfers photo-generated charge from the photo-conversion device to thecharge storage region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a conventional image sensor;

FIG. 2A is a top plan view of a pixel cell according to an embodiment ofthe invention;

FIG. 2B is a cross-sectional view of the pixel cell of FIG. 2 along lineBB′;

FIG. 3 is an exemplary timing diagram for an image sensor according toan embodiment of the invention;

FIG. 4A is a schematic diagram illustrating the location ofphoto-generated charge at a stage of operation of the pixel cell of FIG.2;

FIG. 4B is a schematic diagram illustrating the location ofphoto-generated charge at a stage of operation of the pixel cell of FIG.2;

FIG. 4C is a schematic diagram illustrating the location ofphoto-generated charge at a stage of operation of the pixel cell of FIG.2;

FIG. 5A is a cross-sectional view of the pixel cell of FIG. 2 at aninitial stage of fabrication;

FIG. 5B is a cross-sectional view of the pixel cell of FIG. 2 at anintermediate stage of fabrication;

FIG. 5C is a cross-sectional view of the pixel cell of FIG. 2 at anintermediate stage of fabrication;

FIG. 5D is a cross-sectional view of the pixel cell of FIG. 2 at anintermediate stage of fabrication;

FIG. 5E is a cross-sectional view of the pixel cell of FIG. 2 at anintermediate stage of fabrication;

FIG. 5F is a cross-sectional view of the pixel cell of FIG. 2 at anintermediate stage of fabrication;

FIG. 5G is a cross-sectional view of the pixel cell of FIG. 2 at anintermediate stage of fabrication;

FIG. 5H is a cross-sectional view of the pixel cell of FIG. 2 at anintermediate stage of fabrication;

FIG. 5I is a cross-sectional view of the pixel cell of FIG. 2 at anintermediate stage of fabrication;

FIG. 5J is a cross-sectional view of the pixel cell of FIG. 2 at anintermediate stage of fabrication;

FIG. 5K is a cross-sectional view of the pixel cell of FIG. 2 at anintermediate stage of fabrication; and

FIG. 6 is a schematic diagram of a processing system according to anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof and illustrate specificembodiments in which the invention may be practiced. In the drawings,like reference numerals describe substantially similar componentsthroughout the several views. These embodiments are described insufficient detail to enable those skilled in the art to practice theinvention, and it is to be understood that other embodiments may beutilized, and that structural, logical and electrical changes may bemade without departing from the spirit and scope of the presentinvention.

The terms “wafer” and “substrate” are to be understood as includingsilicon, silicon-on-insulator (SOI), or silicon-on-sapphire (SOS)technology, doped and undoped semiconductors, epitaxial layers ofsilicon supported by a base semiconductor foundation, and othersemiconductor structures. Furthermore, when reference is made to a“wafer” or “substrate” in the following description, previous processsteps may have been utilized to form regions or junctions in the basesemiconductor structure or foundation. In addition, the semiconductorneed not be silicon-based, but could be based on silicon-germanium,germanium, or gallium-arsenide.

The term “pixel” refers to a picture element unit cell containing aphotosensor and transistors for converting electromagnetic radiation toan electrical signal. For purposes of illustration, a representativepixel is illustrated in the figures and description herein, andtypically fabrication of all pixels in an image sensor will proceedconcurrently in a similar fashion.

Referring to the drawings, FIG. 2A is a top plan view of a pixel cell300 according to an exemplary embodiment of the invention and FIG. 2B isa cross-sectional view of the pixel cell 300 along line BB′. Forexemplary purposes pixel cell 300 is shown as a five-transistor (5T)pixel cell 300, but the invention is not limited to a pixel cell havinga specific number of transistors and embodiments having other numbers oftransistors are possible.

Pinned photodiode 320 is a photo-conversion device for accumulatingphoto-generated charge. Adjacent to the pinned photodiode 320 is a gate341 of a shutter transistor for determining an integration time for thepixel cell 300 and for transferring charge from the pinned photodiode320 to a charge storage region. For exemplary purposes the shutter gate341 is a global shutter gate, which operates at a same time as shuttergates of other pixels in an image sensor so that all pixels have equaland concurrent integration times. The invention, however, is not limitedto global shuttering techniques and other shuttering techniques may beused as well.

In the illustrated exemplary embodiment of the invention, there is astorage device, which is a single CCD stage. Typically, a CCD stage is ametal oxide semiconductor (MOS) capacitor. A MOS capacitor can begenerally described as a capacitor formed by a metal or other conductivematerial and a semiconductor material separated by an insulatingmaterial. Typically, the conductive material serves as a gate of the MOScapacitor.

Illustratively, the CCD stage is shown as a buried channel CCD stage 330having a CCD gate 380, which is shown partially overlapping both theshutter gate 341 and a transfer gate 343. CCD gate 380 controls the CCDstage 330 and helps to transfer charge to the CCD stage 330 inconjunction with the global shutter gate. CCD stage 330 stores thecharge until the charge is transferred to a sensing node, which ispreferably a floating diffusion region 305, to be read out. Prior toreadout, the charge is transferred via the CCD gate 380 and a transfergate 343 to the floating diffusion region 305.

CCD stage 330 provides increased charge transfer efficiency for thepixel cell 300 over a conventional pixel cell. As is known in the art, aCCD is capable of providing almost complete charge transfer.Accordingly, almost no charge will be lost when transferred from thepinned photodiode 320 to the floating diffusion region 305 and the pixelcell 300 will have improved charge transfer efficiency. Additionally,the CCD stage 330 reduces charge loss while charge is stored in the CCDstage 330 over time. Near a surface of the substrate 301, charge carriedby, for example, electrons may be lost when electrons recombine withholes. Because CCD stage 330 is a buried channel device, charge ismaintained below the surface of the substrate 301 minimizingrecombination and charge loss.

The floating diffusion region 305 is electrically connected to a resettransistor having a gate 345 and to a gate 347 of a source followertransistor. A source/drain region 307 of the reset transistor isconnected to a supply voltage source V_(dd). The reset transistor resetsthe floating diffusion region 305 to a fixed voltage, V_(dd), before thefloating diffusion region 305 receives photo-generated charge from theCCD stage 330. The source follower transistor receives at its gate 347an electrical signal from the floating diffusion region 305. The sourcefollower transistor is also connected to a row select transistor havinga gate 349 for outputting a signal from the source follower transistorto a column readout line in response to a signal on a row select line.

FIG. 3 is an exemplary timing diagram representing the operation of apixel cell 300 (FIGS. 2A–2B) according to an embodiment of theinvention. FIGS. 4A–4C illustrate the location of photo-generated charge444 at stages of operation of pixel cell 300. As shown in FIG. 2A, gate341 receives global shutter (GS) signals, CCD gate 380 receives chargecoupled device (CCD) signals, gate 343 receives transfer (TX) signals,gate 345 receives reset (RST) signals, and gate 349 receives row (ROW)signals. All of these signals could be provided with circuitry as inFIG. 1, by appropriate modification of timing and control circuitry 195,which controls these signals. Supply voltage V_(dd) and otherconnections for gate 347 and for readout are made at connection points303.

Prior to the occurrence of signals shown in FIG. 3, the pinnedphotodiode 320 collects photo-generated charge 444 in response toexternal incident light, as shown in FIG. 4A. After an integration time,a global shutter (GS) signal is pulsed high causing the gate 341 of theshutter transistor to turn on and transfer the photo-generated charge444 from the pinned photodiode 320 to the CCD stage 330. Also at thistime, a CCD signal is pulsed high to turn on CCD gate 380. The CCDsignal stays high and CCD gate 380 remains on to store the charge 444 inthe CCD stage 330, as shown in FIG. 4B.

While the charge 444 is stored by the CCD stage 330, a RST signal ispulsed high causing the gate 345 of the reset transistor to turn on toreset the floating diffusion region 305 to V_(dd). Also at this time, aROW signal turns on the gate 439 of the row select transistor. The resetvoltage on the floating diffusion region 305 is applied to the gate ofthe source follower transistor to provide a current based on the resetvoltage which passes through the row select transistor to a column line.This current is translated into a reset voltage, V_(rst), by readoutcircuitry (not shown) and read out. When readout is completed, the RSTand ROW signals transition to low.

Next, a TX signal is pulsed high and the CCD signal remains high, totransfer the photo-generated charge 444 from the CCD stage 330 to thefloating diffusion region 305. Once the charge 444 is transferred to thefloating diffusion region 305, as shown in FIG. 4C, the TX and CCDsignals pass to low.

Also, at this time a ROW signal again turns on the gate 349 of the rowselect transistor. The photo-generated charge 444 on floating diffusionregion 305 is applied to the gate of the source follower transistor tocontrol the current passing through row select transistor. This currentis similarly translated into a voltage, V_(sig), and read out. When asignal indicating the photo-generated charge 444 from the floatingdiffusion region 305 is read out, the ROW signal transitions to low.

The fabrication of pixel cell 300 is described below with reference toFIGS. 5A through 5K. No particular order is required for any of theactions described herein, except for those logically requiring theresults of prior actions. Accordingly, while the actions below aredescribed as being performed in a general order, the order is exemplaryonly and may be altered.

FIG. 5A illustrates a pixel cell 300 at an initial stage of fabrication.Substrate 301, is illustratively of a first conductivity type, which,for this exemplary embodiment is p-type. Isolation regions 302 areformed in the substrate 301 and filled with a dielectric material. Thedielectric material may be an oxide material, for example a siliconoxide, such as SiO or SiO₂; oxynitride; a nitride material, such assilicon nitride; silicon carbide; a high temperature polymer; or othersuitable dielectric material. As shown in FIG. 5A, the isolation region302 can be a shallow trench isolation (STI) region and the dielectricmaterial is preferably a high density plasma (HDP) oxide, a materialwhich has a high ability to effectively fill narrow trenches.

As shown in FIG. 5B, a first insulating layer 340 a of silicon oxide isgrown or deposited on the substrate 301. The layer 340 a will be thegate oxide layer for the subsequently formed transistor gates. Firstinsulating layer 340 a may have a thickness of approximately 50Angstroms (Å). Next, a layer of conductive material 340 b is depositedover the oxide layer 340 a. The conductive layer 340 b will serve as thegate electrode for the subsequently formed transistors. Conductive layer340 b may be a layer of polysilicon, which may be doped to a secondconductivity type, e.g. n-type, and may have a thickness ofapproximately 1000 Å. A second insulating layer 340 c is deposited overthe polysilicon layer 340 b. The second insulating layer 340 c may beformed of an oxide (SiO₂), a nitride (silicon nitride), an oxynitride(silicon oxynitride), ON (oxide-nitride), NO (nitride-oxide), or ONO(oxide-nitride-oxide). Second insulating layer 340 c may have athickness of approximately 1000 Å.

The layers, 340 a, 340 b, and 340 c, may be formed by conventionaldeposition methods, such as chemical vapor deposition (CVD) or plasmachemical vapor deposition (PECVD), among others. The layers 340 a, 340b, and 340 c are then patterned and etched to form the multilayer gatestack structures 341, 343, and 345 shown in FIG. 5C. The gate stack 341is the gate structure for a global shutter transistor, the gate stack343 is the gate structure for a transfer transistor, and gate stack 345is the gate structure for a reset transistor.

The invention is not limited to the structure of the gates 341, 343, and345 described above. Additional layers may be added or the gates 341,343, and 345 may be altered as is desired and known in the art. Forexample, a silicide layer (not shown) may be formed between the gateelectrodes 340 b and the second insulating layers 340 c. The silicidelayer may be included in the gates 341, 343, and 345, or in all of thetransistor gate structures in an image sensor circuit, and may betitanium silicide, tungsten silicide, cobalt silicide, molybdenumsilicide, or tantalum silicide. This additional conductive layer mayalso be a barrier layer/refractor metal, such as TiN/W or W/N_(x)/W, orit could be formed entirely of WN_(x).

A p-well 304 is implanted into substrate 301 as shown in FIG. 5D. P-well304 is formed in the substrate 301 from a point below the shutter gate341 to a point below the STI region 302 that is on a side of the resetgate 345 opposite to the transfer gate 343. P-well 304 may be formed byknown methods. For example, a layer of photoresist (not shown) may bepatterned over the substrate 301 having an opening over the area wherep-well 304 is to be formed. A p-type dopant, such as boron, may beimplanted into the substrate through the opening in the photoresist.Illustratively, the p-well 304 is formed having a p-type dopantconcentration that is higher than adjacent portions of the substrate301.

Doped regions 320 a and 330 a of a second conductivity type areimplanted in the substrate 301 for the pinned photodiode 320 and CCDstage 330, respectively, as shown in FIG. 5E. Pinned photodiode region320 a, and CCD stage region 330 a are illustratively lightly dopedn-type regions. The pinned photodiode and CCD stage regions 320 a and330 a may be formed by methods known in the art. For example, a layer ofphotoresist (not shown) may be patterned over the substrate 301 havingan opening over the surface of the substrate 301 where the pinnedphotodiode and CCD stage regions 320 a and 330 a are to be formed. Ann-type dopant, such as phosphorus, arsenic, or antimony, is implantedthrough the opening and into the substrate 301. Multiple implants may beused to tailor the profile of the regions 320 a and 330 a. If desired,an angled implantation may be conducted to form the pinned photodiodeand CCD stage regions 320 a and 330 a, such that implantation is carriedout at angles other than 90 degrees relative to the surface of thesubstrate 301.

The pinned photodiode region 320 a is on an opposite side of the shuttergate 341 from the CCD stage region 330 a and is approximately alignedwith an edge of the shutter gate 341 forming a photosensitive chargeaccumulating region for collecting photo-generated charge. The CCD stageregion 330 a is between and approximately aligned with an edge of theshutter gate 341 and an edge of the transfer gate 343 forming a storageregion for storing photo-generated charge.

As shown in FIG. 5F, lightly doped drain (LDD) implants are performed byknown techniques to provide LDD regions 305 a and 307 a. LDD region 305a is implanted between transfer gate 343 and reset gate 345 and isapproximately aligned with respective edges of transfer gate 343 andreset gate 345. LDD region 307 a is also approximately aligned with anedge of the reset gate 345, but is implanted adjacent to the reset gate345 on a side of the reset gate 345 opposite to the transfer gate 343.For exemplary purposes LDD regions 305 a and 307 a are lightly dopedn-type regions.

FIG. 5G depicts the formation of a layer 342, which will subsequentlyform sidewall spacers on the sidewalls of the gates 341, 343, and 345.Illustratively, layer 342 is an oxide layer, but layer 342 may be anyappropriate dielectric material, such as silicon dioxide, siliconnitride, an oxynitride, ON, NO, ONO, or TEOS, among others, formed bymethods known in the art. Layer 342 may have a thickness ofapproximately 700 Å.

Doped surface layers 320 b and 330 b for the pinned photodiode 320 andthe CCD stage 330, respectively, are implanted, as illustrated in FIG.5H. Doped surface layers 320 b and 330 b are doped to a firstconductivity type, which for exemplary purposes is p-type. Doped surfacelayers 320 b and 330 b may be highly doped p+ surface layers. A p-typedopant, such as boron, indium, or any other suitable p-type dopant, maybe used to form p+ surface layers 320 b and 330 b.

The p+ surface layers 320 b and 330 b may be formed by known techniques.For example, layers 320 b and 330 b may be formed by implanting p-typeions through openings in a layer of photoresist. Alternatively, layers320 b and 330 b may be formed by a gas source plasma doping process, orby diffusing a p-type dopant into the substrate 301 from an in-situdoped layer or a doped oxide layer deposited over the area where layers320 b and 330 b are to be formed.

As shown in FIG. 5I, a dry etch step is conducted to etch the oxidelayer 342, with the remaining parts of layer 342 forming sidewallspacers 342 on the sidewalls of gates 341, 343, and 345.

An insulating layer 381 is deposited by known methods over the substrate301 and over gates 341, 343, and 345, as shown in FIG. 5J. Insulatinglayer 381 may have a thickness of approximately 100 Å. Illustratively,insulating layer 381 is a layer of silicon nitride (Si₃N₄), but otherappropriate dielectric materials may be used.

A conductive layer 382 is deposited by known methods over Si₃N₄ layer381. Conductive layer 382 may have a thickness of approximately 1000 Å.Illustratively, conductive layer 381 is a layer of p-type polysilicon,but other appropriate conductive materials may be used. Layers 381 and382 are patterned and etched to form CCD gate 380, as shown in FIG. 5K.

Source/drain regions 305 and 307 may be implanted by known methods toachieve the structure shown in FIG. 3B. Source/drain regions 305 and 307are formed as regions of a second conductivity type, which for exemplarypurposes is n-type. Any suitable n-type dopant, such as phosphorus,arsenic, or antimony, may be used to form source/drain regions 305 and307. Source/drain region 305 is formed between transfer gate 343 andreset gate 345; and source/drain region 307 is formed adjacent to resetgate 345 on a side of reset gate 345 opposite to transfer gate 343.

Conventional processing methods may be used to complete the pixel cell300. For example, insulating, shielding, and metallization layers toconnect gate lines and other connections to the pixel cell 300 may beformed. Also, the entire surface may be covered with a passivation layer(not shown) of, for example, silicon dioxide, BSG, PSG, or BPSG, whichis CMP planarized and etched to provide contact holes, which are thenmetallized to provide contacts. Conventional layers of conductors andinsulators may also be used to interconnect the structures and toconnect pixel cell 300 to peripheral circuitry.

While the above embodiments are described in connection with theformation of pnp-type photodiodes the invention is not limited to theseembodiments. The invention also has applicability to other types ofphotodiodes and to photodiodes formed from npn regions in a substrate.If an npn-type photodiode is formed the dopant and conductivity types ofall structures would change accordingly, with the transfer and shuttergates being part of PMOS transistors, rather than NMOS transistors as inthe embodiments described above.

Although the invention is described in connection with a five-transistor(5T) pixel cell, the invention may also be incorporated into other CMOSpixel cell designs having different numbers of transistors. Withoutbeing limiting, such a design may include a six-transistor (6T) pixelcell. A 6T pixel cell differs from the 5T cell by the addition of atransistor, such as an anti-blooming transistor.

According to an embodiment of the invention, one or more pixel cells 300as described above in connection with FIGS. 3–5K may be part of an arrayof pixel cells. Such an array may be part of an image sensor similar tothe image sensor described above in connection with FIG. 1.

FIG. 6 shows a typical processor-based system 677 including an imagesensor 699 having an array of pixel cells, wherein one or more of thepixel cells are formed as described above in connection with FIGS. 3–5K.A processor-based system 677 is exemplary of a system having digitalcircuits that could include image sensors. Without being limiting, sucha system could include a computer system, camera system, scanner,machine vision, vehicle navigation, video phone, surveillance system,auto focus system, star tracker system, motion detection system, imagestabilization system, and data compression system.

Processor-based system 677, which for exemplary purposes is a computersystem, generally comprises a central processing unit (CPU) 670, such asa microprocessor, that communicates with an input/output (I/O) device675 over a bus 673. The image sensor 699, which produces an image outputfrom a pixel array, also communicates with the system 677 over bus 673.The processor-based system 677 also includes random access memory (RAM)676, and may include peripheral devices, such as a floppy disk drive 671and a compact disk (CD) ROM drive 672, which also communicate with CPU770 over the bus 673. The image sensor 699 may be combined with aprocessor, such as a CPU, digital signal processor, or microprocessor,with or without memory storage on a single integrated circuit or on adifferent chip than the processor.

It is again noted that the above description and drawings are exemplaryand illustrate preferred embodiments that achieve the objects, featuresand advantages of the present invention. It is not intended that thepresent invention be limited to the illustrated embodiments. Anymodification of the present invention which comes within the spirit andscope of the following claims should be considered part of the presentinvention.

1. A pixel cell comprising: a photo-conversion device that generatescharge; a gate controlled charge storage region that stores the chargeunder control of a control gate, a first transistor having its gatebetween the photo-conversion device and the charge storage region fortransferring charge from the photo-conversion device to the chargestorage region; a sensing node; and a second transistor having its gatebetween the charge storage region and the sensing node.
 2. The pixelcell of claim 1, wherein the sensing node is a floating diffusionregion.
 3. The pixel cell of claim 1, wherein the control gate at leastpartially overlaps the first and second transistor gates.
 4. A pixelcell comprising: a photo-conversion device that generates charge; a gatecontrolled charge storage region that stores the charge under control ofa control gate, wherein the charge storage region comprises a dopedregion of a second conductivity type and a doped surface layer of afirst conductivity type over and in contact with the doped region of asecond conductivity type, and wherein the control gate is over the dopedsurface layer; a first transistor having its gate between thephoto-conversion device and the charge storage region for transferringcharge from the photo-conversion device to the charge storage region; asensing node; and a second transistor having its gate between the chargestorage region and the sensing node.
 5. The pixel cell of claim 4,wherein the control gate overlaps the first and second transistor gates.